Very high frequency omnidirectional ranging (VOR) systems have been employed for many years to provide bearing information in various applications. In a conventional VOR system used for aircraft navigation, a transmitted signal emanating from a ground antenna is effectively directed around a circular path at a rate of 30 times per second. A 9,960 Hz tone, which is frequency modulated by a 30 Hz reference tone, modulates the amplitude of a continuous wave signal to form the transmitted signal. An aircraft receiving the transmitted signal computes a bearing quantity based upon the phase difference between the 30 Hz reference tone and the 30 Hz amplitude modulation provided by the rotating directivity of the antenna.
In a Doppler VOR system used for aircraft navigation, an effective source location of a ground antenna pattern repeatedly cycles around a closed, planar path. In practice, a circular array of antennas is employed at a ground station, wherein each antenna is sequentially connected to a transmitter in order to simulate the rotation of a single antenna around the circle at a rate of 30 times per second. An aircraft receives a Doppler-shifted signal having a phase angle proportional to the bearing of the aircraft. By further receiving a transmitted reference signal containing a 30 Hz reference tone, the aircraft can determine the bearing.
U.S. Pat. No. 3,999,187 to Johnson discloses a Doppler VOR beacon having a plurality of omnidirectional antennas disposed around the periphery of a circle centered on a central antenna. Further included are means for commutating a source of sideband radiation around the plurality of omnidirectional antennas to simulate a rotating source, and means for radiating from the central antenna carrier power modulated with a reference wave.
U.S. Pat. No. 4,591,861 to Kautz discloses a double-sideband Doppler VOR system having plurality of antennas arranged equidistantly on a circle. Two opposite antenna pairs are simultaneously activated by a commutator in a manner to simulate a uniform rotation of an active antenna.
Other patents which disclose Doppler VOR systems employing a circular arrangement of transmitting antennas include U.S. Pat. No. 3,972,044 to Alford, U.S. Pat. No. 3,988,733 to Kohler, U.S. Pat. No. 4,382,259 to Becavin et al., U.S. Pat. No. 4,417,250 to Grousseau, and U.S. Pat. No. 4,434,423 to Kautz.